fix: scope paragraph reorder updates to document

[zhonggang666]

Summary

• Scope paragraph reorder range updates to the current document.
• Validate the moved paragraph against the requested knowledge/document path before adjusting positions.

Root cause

adjust_position shifted paragraphs only by position range, so documents sharing the same position values could be updated together.

Validation

• python -m py_compile apps\knowledge\serializers\paragraph.py
• git diff --check

Fixes #6373